Given the choice, most patients with hypothyroidism would prefer to take a thyroid hormone product that includes both T3 and T4. This isn’t just my observation: it’s the conclusion of a landmark study published in the New England Journal of Medicine on February 11, 1999. In this ten-week study, patients with hypothyroidism were randomized into two groups. One group received isolated T4 preparations for the first five weeks and a combination of T3 and T4 for the last five weeks; in the second group, this sequence was reversed. All of the capsules looked alike, so the patients were unaware of which treatment they were receiving during each five-week period.

On the last day of each five-week period, patients were administered standardized psychological tests to assess their levels of depression, anxiety, anger, and other traits. They were also given cognitive tests of memory, attention, learning, and other functions. On eleven of seventeen measures of mood and cognition, there was no significant difference between the two treatments. However, on six measures, the combination of T3 and T4 proved superior to isolated T4. In particular, when patients received both thyroid hormones, their symptoms of fatigue, depression, and anger were significantly improved, and they performed better on tests of attention, mental flexibility, and learning.

In addition to performing better on standardized tests, patients rated their own mood and physical symptoms as significantly improved on the combination product in comparison to isolated T4. When asked which treatment they preferred, the majority preferred the combination product, stating that they had more energy, could concentrate more easily, and simply felt better.